Optical fibres commonly comprise a glass core, inside which the transmitted optical signal is confined by the surrounding cladding (typically with a diameter of about 120-130 μm), generally made of glass. The combination of core and cladding is usually identified as “optical waveguide”. The optical waveguide is generally protected by a coating, typically of polymeric material, which protects the fibre glass from the external environment and provides resistance to physical handling forces, such as those encountered when the fibre is subjected to cabling operations. The coating typically comprises a first coating layer positioned in direct contact with the cladding, also known as the “primary coating”, and at least one second coating layer, also known as “secondary coating”, surrounding the first coating. In the art, the combination of primary coating and secondary coating is sometimes also identified as “primary coating system”, as both these layers are generally applied during the drawing manufacturing process of the fibre. In this case, the coating in contact with the cladding is called “inner primary coating” while the coating in contact with and surrounding the inner primary coating is called “outer primary coating”. In some instance, a single coating can be applied in contact with the cladding. Thereafter, the term “primary coating” shall designate the inner primary coating and the term “secondary coating” shall designate the outer primary coating.
Generally, the primary coating is made of a relatively soft material having a relatively low modulus of elasticity E′ at room temperature (typically of from 0.1 MPa to 5 MPa) and a low glass transition temperature (Tg), for example lower than −20° C. The secondary coating is generally formed by a more rigid polymer having a higher modulus of elasticity E′ at room temperature (typically of from 500 MPa to 2000 MPa) and a higher Tg compared to those of the primary coating layer.
For certain applications, optical waveguides may be coated with a single coating layer having modulus of elasticity and Tg values which are intermediate between those of the primary coating and the secondary coating. The overall diameter of the optical waveguide with the primary and secondary coating can be of from 150 μm to 250 μm.
The polymer materials generally used to form primary coatings, secondary coatings and single layer coatings are obtained from compositions comprising acrylate oligomers and monomers that are cured by means of UV radiation in the presence of a suitable photoinitiator. The acrylate polymer coatings, however, should be formed on the optical waveguide at relatively low temperatures, e.g. from ambient temperature to about 50° C., and cured in the presence of an inert atmosphere (e.g. under nitrogen gas) in order to avoid the thermal degradation of the polymer materials and guarantee the proper adhesion of the coating layer to the optical waveguide or to the primary coating. These constraints require the use of special apparatuses for controlling the temperature during the polymer deposition and curing process. Typically, radiation curing ovens are continuously flushed with inert gases (e.g. nitrogen or helium) in order to maintain the required conditions.
The need for the above-described stringent operating conditions, apparently, makes the manufacturing process of the optical fibres and the apparatus used for carrying out the process thereof rather complex and costly.
A thermally curable coating for optical fibres is disclosed in WO2018/015790. This patent application describes an optical fibre comprising an optical waveguide surrounded by a coating comprising a cured polymer material comprising a polyester obtained by esterification of: a reactant A selected from an acid, a triglyceride, or a mixture of triglycerides, having a C16-C24 aliphatic chain comprising at least two conjugated double bonds; and a polyol made of at least one monomer comprising at least 3 hydroxyl groups, the polyol being thermally stable up to 300° C. (reactant B). The mechanical properties of the coating material are especially suitable for use as primary or single coating.